In recent memory hard disk drives, there is a demand for decreasing a flying height of a magnetic head so as to enhance a recording density, thereby reducing a unit recording area, for the purpose of achieving a high capacity and a small diameter. In connection with this, even in a process of producing a hard disk, the surface quality required of a polished surface obtained by polishing an object to be polished is increasing. That is, in accordance with the reduction in flying height of the head, the surface roughness and the micro waviness of the polished surface obtained by polishing need to be reduced, and it is necessary to reduce roll-off (edge rounding of the end side of a substrate) and the number of protrusions. It also is necessary to reduce the number of scratches on the polished surface in accordance with the reduction in a unit recording area, and further, the size and depth of the scratches need to be decreased.
The recording system in a hard disk is shifting from a horizontal magnetic recording system (which also is called an in-plane magnetic recording system) to a perpendicular magnetic recording system along with the request for an increase in a capacity.
The process of producing a horizontal magnetic recording hard disk includes a substrate formation step and a media step. In the substrate formation step, a base is subjected to a polishing treatment and a cleaning treatment in this order a plurality of times, whereby a substrate for a hard disk is produced. In the media step, small unevenness is provided to both principal surfaces of a substrate for a hard disk by polishing (texture step), the substrate is cleaned (cleaning step), and magnetic layers are formed respectively on both principal surfaces of the substrate (magnetic layer formation step). In the texture step, particles adhering to the substrate for a hard disk in the substrate formation step are removed by polishing. In the case where the substrate for a hard disk is, for example, an aluminum substrate having Ni—P containing layers as outermost layers (hereinafter, which may be abbreviated as a “substrate having Ni—P containing layers”), when a cleaning composition with high acidity is used, the Ni—P containing layer is dissolved (corroded) excessively. For this reason, in the cleaning step, only water or an alkaline nonionic surfactant has been used (see, for example, Patent Document 1).
Patent Document 1: JP 2001-003081 A